1. Field of the Invention
This invention relates to microwave waveguides and more particularly, to coupling windows in such waveguides which are capable of propagating high frequency, high power microwave power without overheating, significant mode conversion, or excessive sensitivity to frequency.
2. Description of the Prior Art
A waveguide window in a microwave power system generally permits power to pass from one waveguide to a second waveguide, but presents a physical barrier between the two waveguides. The waveguides may contain different gases or have different pressure levels, and one or both waveguides may be evacuated. For example, in high power microwave vacuum devices, such as gyrotrons and the like, power is generally transferred between an evacuated chamber or waveguide in the device and a waveguide having a gaseous environment through one or more waveguide windows which provide a hermetic seal between the two media. Also, in fusion reactors where microwave power in the HE.sub.1,1 mode is added to a plasma, such a barrier is desirable near the reactor to confine the constituents of the plasma.
A window of the prior art may consist of a dielectric disk secured in a waveguide. At low frequencies and low power, the heat generated due to inherent dielectric loss may be dissipated by cooling the edge of the disk. However, for a given material, frequency and temperature rise, there is an absolute limit on the allowable power, independent of window size. This limit drops rapidly with increasing frequency. If the additional heat is not adequately dissipated, the window may overheat and fail.
Another known window has a pair of disks with a dielectric liquid flowing between them for cooling. At high frequency or high power, the coolant flow rate must increase, which requires increased pressure and thicker disks to withstand the pressure. The increased thickness of the window further increases the total dissipation. If the disks become several wavelengths thick, they become very frequency sensitive, particularly when used in microwave tubes, and sensitive to variations in the dielectric constant of the disk material. In addition, the heat transfer through the thick disks is poor.
A plurality of apertures in a common wall can be used for transferring microwave power between waveguides having equal phase velocities as discussed in S. E. Miller, Coupled Waveguide Theory and Application Bell System Tech. J., May 1954, p. 661-719. Since the power through each coupling aperture can be made small, if dielectric plugs are used to seal each aperture, the plugs may be edge cooled, and the plurality of apertures may transfer high power in the megawatt range. However, at the frequencies and powers of interest here, the waveguides must be over moded to have acceptably low loss; that is, many modes can propagate in the waveguides, and generally efficient transfer of power between waveguides requires a highly symmetric geometry.
A coupling apparatus using a plurality of relatively small apertures is disclosed in C. P. Moeller, U.S. Pat. No. 4,523,127. The described apparatus is constructed of a first cylindrical waveguide having a second cylindrical waveguide mounted coaxially therein forming an inner cylindrical waveguide and an outer coaxial waveguide. The inner waveguide has a plurality of apertures about its surface for coupling microwave power between the inner (cylindrical) and outer (coaxial) waveguides. This arrangement has the required azimuthal symmetry, but the different cross-sectional types (cylindrical and coaxial) of the two circular waveguides makes phase velocity matching difficult as a practical matter because the apertures tend to perturb the two waveguides dissimilarly. Also, the only low loss modes suitable for this geometry, the circular TE.sub.o,n modes, require further conversion to the HE.sub.1,1 mode before the power is launched into a plasma.
It is frequently desirable to add power to a plasma by radiating high power microwaves into that plasma. The HE.sub.1,1 mode has low loss and an ideal radiation pattern for launching high power microwaves at high frequencies into a plasma. To be propagated in circular waveguide, the HE.sub.1,1 mode requires corrugations around the inner surface of the waveguides. These corrugations interfere, however, with the wall currents which provide the coupling when apertures are introduced between two waveguides. As disclosed in J. L. Doane Int. Journal of Infrared and Millimeters Waves, Vol. 8, p. 13 (1987), HE.sub.1,1 mode waves can be conveyed with low loss in rectangular waveguides comprising two opposed corrugated surfaces and two smooth surfaces. No coupling window is known, however, for efficiently coupling HE.sub.1,1 mode microwaves between waveguides using phase velocity coupling.
A need exists for a waveguide coupling apparatus to couple HE.sub.1,1 mode microwaves between a pair of waveguides at high power levels which apparatus avoids mode conversion and exhibits substantially the same phase velocity in both coupled waveguides.